1. Field of the Invention
This invention relates to a process for producing a toner to be used in electrophotography, electrostatic photography, magnetic recording, electrostatic printing, etc.
2. Description of the Prior Art
The operation of so-called "fixing" has been practiced in the art when it is desired to store an image of colored fine powder called "toner", which image has been obtained by developing an electrostatic or magnetic latent image. As such fixing methods, a method in which toner on a latent image bearing surface as such or after being transferred onto a transfer material such as paper is heated in a heat chamber to melt and embed the toner simultaneously; a method in which toner is dissolved with the use of a solvent to effect adhesion of the toner and thereafter the solvent is removed; a method in which a resin solution called a fixing liquor is applied and fixed on the image; and other methods, have been known.
On the other hand, as a developing method for visualization of electric latent images with toner, a large number of methods have been known such as the magnetic brush method disclosed in U.S. Pat. No. 2,874,063, the cascade developing method disclosed in U.S. Pat. No. 2,618,552, the powder cloud method disclosed in U.S. Pat. No. 2,221,776, the fur brush method and the liquid developing method. The toner to be used in these developing and fixing methods is constituted so as to satisfy the various developing and fixing methods as mentioned above and give sufficient performance. For such purposes, it has been practiced in the art to mix a colorant such as carbon black or iron oxide with a binding material such as polystyrene or phenol resin, crushing the mixture into minute powder, and after carrying out optionally the operation of classification, etc., mixing the powder with carrier particles such as a magnetic material or glass beads or an electroconductive agent, if desired depending on the respective developing method, to be provided for use.
As toner to be used in such a form, various kinds of toner have been known in the art. They are constituted so as to be adapted for developing methods and fixing methods as described above. Toner is constituted in a form so as to satisfy such developing characteristic and fixing characteristic, but it is generally difficult for a toner to have well-balanced developing characteristic and fixing characteristic.
This is because there are essential properties required for a toner which are basically incompatible with each other, namely an ability of adhesion and agglomeration required for fixing and or ability of individual and independent movement of toner particles required for development.
In particular, such essential but basically incompatible properties are more severely required recently when there are increasing demands for a high performance toner, a toner capable of developing and fixing at high speed and a toner which can be fixed with a low energy (e.g. very slight pressure). Thus, if a toner having a better fixing characteristic is desired, it becomes more difficult to give a satisfactory developing characteristic to the toner.
For overcoming such a contradiction, a toner in the form of a capsule having developing characteristic and fixing characteristic separated into a shell and a core, respectively, is also proposed. However, practically, it is not necessarily easy to prepare an ultra-fine particulate form of capsule adapted for toner.
Also, as described above, the toner of the prior art has been generally produced by melt-mixing a thermoplastic resin with a colorant such as a dye or a pigment at a high temperature, cooling the resultant mixture to room temperature and crushing the cooled product into fine particles. However, the toner produced according to such a process has no constant shape or size on account of the nature of the process, and it is also essentially difficult to obtain uniformness between toner particles as well as homogenity in each toner particle. In sufficiency in practical performance caused by such shortcomings has appeared in various forms.
Production of the toner according to such a process also requires a large amount of cost. In particular, the step for preparing minute particles has required a large amount of energy, thus taking a large part of the production cost in respect of both installation cost and running cost.
According to such a process, there has also been a great restriction with respect to the materials to be employed. For example, for obtaining a fine particulate toner with a homogeneous composition, kneading must be done to a considerable extent of homogenity, whereby a material adapted for kneading must be used.
On the other hand, for obtaining a fine powdery toner, crushing characteristic of the materials has posed a great problem in productivity. More specifically, to be made into a toner, the binder used therefore is first required to have fragility, which, however, did not coincide with the characteristics of the binder required from the aspect of the toner performance. In other words, it has not been possible to use a binder having desired characteristics only from the standpoint of the requisite performance of a toner in the process of the prior art.
In order to solve these problems, a process utilizing so-called emulsion polymerization is also proposed, in which particles are formed simultaneoulsly with polymerization to give toner particles. However, when a toner is produced by such a process, it is difficult to completely remove the emulsifier or dispersant employed for stabilization of dispersion, and therefore a toner with sufficient performances could hardly be obtained.